In typical cooling systems for cooling the engine or the like of a work machine, a cooling fan is driven by an electric or hydraulic motor and the rotational speed of the cooling fan is controlled independently of the control of the rotation of the engine. One example of such cooling systems is disclosed in Japanese Kokai Patent Publication No. 10-68142.
As shown in FIG. 5, the cooling system disclosed in Japanese Kokai Patent Publication No. 10-68142 includes a radiator 52 and an oil cooler 53 which are disposed separately from an engine 51; a cooling fan 54 for forcibly cooling the radiator 52 and the oil cooler 53; and a fan driving circuit 55 for forwardly or reversely rotating the cooling fan 54 or stopping the rotation of it. The fan driving circuit 55 has a hydraulic pump 56 rotated by the engine 51; a forwardly and reversely rotatable hydraulic motor 57 operated by hydraulic oil supplied from the hydraulic pump 56; and an electromagnetic selector valve 60 interposed in a hydraulic pipe line 59 for connecting the hydraulic pump 56 and a tank 58 to the two ports of the hydraulic motor 57. Switching of the electromagnetic selector valve 60 is controlled by a control unit 64 based on detection data from an air temperature probe 61, a coolant temperature sensor 62 and an hydraulic oil temperature sensor 63. In a case where the temperature of air, the temperature of the coolant and the temperature of the hydraulic oil are lower than their settings at the time of starting up the engine, the controller 64 keeps the cooling fan stopped even if the engine starts up. If the temperature of the coolant rises to its setting or higher while the temperature of the hydraulic oil remains lower than its setting, the controller 64 controls the cooling fan 54 to reversely rotate, thereby warming the hydraulic oil within the oil cooler 53 by warm air which has passed through the radiator 52 and removing dust caught in the radiator 52 and others by a reverse current of air. If the temperature of the coolant and the temperature of the hydraulic oil both rise to their respective settings or higher, the controller 64 controls the cooling fan 54 to forwardly rotate, thereby cooling both the coolant and hydraulic oil. With this arrangement, the time required for warm up driving of the hydraulic equipment is reduced and improved cooling efficiency is achieved by removal of dust caught in the radiator 52 and the oil cooler 53.
Another cooling system similar to the above is disclosed in Japanese Kokai Patent Publication No. 11-193719. The cooling system of Japanese Kokai Patent Publication No. 11-193719 is such that a first driven state wherein the cooling fan is forwardly rotated for a first set period to cool the radiator and a second driven state wherein the cooling fan is reversely driven for a second set period to carry out dust removal from the radiator are regarded as one interval and this interval is repeated. In this system, if the temperature of the coolant is higher than its setting, the second driven state is maintained for a third set period whereby the dust catching net is prevented from being clogged before the performance of cooling the engine drops.
The above-described prior art systems have revealed the problem that the rotating direction of the cooling fan is sometimes switched while the cooling fan is rotating at high speed and this may cause a peak pressure in the fan driving circuit resulting in an increase in the load imposed on the driving circuit and occurrence of abnormal noise. In addition, a switchover spool (selector valve) having a neutral position is necessary for changing the rotating direction of the cooling fan during high-speed rotation, which unavoidably entails an increase in the cost of the system.
The invention is directed to overcoming these problems and a primary object of the invention is therefore to provide a cooling system for use in a work machine, in which an increase in the load imposed on the fan driving circuit and occurrence of abnormal noise can be prevented without fail by means of an inexpensive arrangement, at the time of switching the rotating direction of the cooling fan.